Electrical machines needs cooling to dissipate heat, which is generated during its operation by an ohmic resistance, by iron hysteresis, etc.
When a machine is installed indoor at a dry atmosphere it is possible to operate the machine without a housing, so a cooling is achieved by the circulation of ambient air through the machine.
But when the machine is installed under harsh conditions, like it is for generators being used in offshore wind-turbines, the electrical machine need to be totally enclosed, so ambient air is not allowed to circulate through the machine. For this application dedicated cooling-methods are required.